The study measured the testosterone levels of 600 participants when they were 21-years-old and then again when they were 25. Men with high testosterone levels were more likely to find a mate and father a child. After having a child, though, their testosterone levels dropped significantly. And the more time these fathers spent with their children, the lower their testosterone level was. Single men, however, had a steady level of testosterone after four years.

The researchers suggest this physiological reaction is beneficial to the father as he needs to be more nurturing and less aggressive after having a child. Though he may not like the idea of losing some of his testosterone-fueled manhood, these changes may help him become a better father. [Scientific American; Image from AnnaKostyuk/Shutterstock]